Method and apparatus for continuous gas liquid reactions

ABSTRACT

A method and apparatus for improving contact between a liquid and a reactant gas by moving the liquid in a serpentine path that moves horizontally and vertically through individual chambers or stations in an elongated tank or reactor and introducing a reactant gas into the liquid in one or more of the stations or chambers as the liquid moves through the tank or reactor.

This is a divisional application of U.S. Ser. No. 09/293,614, filed Apr.16, 1999, now U.S. Pat. No. 6,451,268.

BACKGROUND OF THE INVENTION

The present invention pertains to continuous reactor processes and inparticular the use of such reactors to effect reaction between a liquidand a reactant gas.

In the manufacture of precipitated calcium carbonate it is conventionalto use a batch reactor, a continuously stirred tank reactor (CSTR) or apipe-line-type plug flow (PF) reactor to contact a liquid slurry ofwater and calcium hydroxide with carbon dioxide in order to synthesizeprecipitated calcium carbonate having particular characteristics.

Continuous stirred tank reactors rely upon a mechanical agitator and theintroduction of the reactant gas directly into the liquid to achieve thedesired reaction. The continuous stirred tank reactor is operated atpredetermined temperatures, pressures and agitation rates in accord withthe product being produced by the contact of a liquid with a reactantgas. Continuous stirred tank reactors are generally limited in size. Inorder to achieve increased system throughput or economics of scale,multiple reactors must be employed.

The plug flow reactor is generally a long tubular shape reactor filledwith the liquid which is generally moving in a straight line directioninto which the reactant gas is introduced. Plug flow reactors aregenerally expensive since they require a long pipe line and the use of ahigh purity gases in certain applications. Two reasons for using highpurity gas are, to avoid slugging and to enable the use of smaller sizepipe.

Numerous techniques have been used to produce precipitated calciumcarbonate having a controlled particle size for use in variousapplications and in particular the treatment of papers.

U.S. Pat. No. 2,538,802 discloses and claims a continuous process forproducing precipitated calcium carbonate having a desired particle sizerange using a two-stage dual carbonator system. [Patentees give detailsof other reactors that were available at the time, i.e. prior to 1951.]

U.S. Pat. No. 3,150,926 discloses and claims a continuous process forproducing precipitated calcium carbonate using an elongated reactorhaving dual screw type conveyors to move the slurry from the entry endto a discharge end of the reactor. Paddles and longitudinal blades areused to move the materials through the reactor in what patenteesdescribe as a flow pattern “likened it to a rock and curve-bound streamwherein the stream flow is basically in one direction although theobstacles and curves create back flows, eddys and swirls which slow therate of flow while keeping the entire stream in a constant state ofagitation.” Patentees also described the action as that of a“mechanically fluidized bed.” The reactor is enclosed and carbon dioxideis introduced through the bottom of the reactor in what is called thecarbonation zone.

U.S. Pat. No. 4,133,894 discloses and claims a multi-step, multi-vesselprocess for preparing precipitated calcium carbonate having less that a0.1 μm particle size. Various processing parameters are disclosed.

U.S. Pat. No. 4,888,160 discloses and claims using a stirred tankreactor for preparing various precipitated calcium carbonate products.The Patent discloses control of various parameters, e.g. pH, compositionof the slurry, temperature, reacting gas purity, and the use ofinhibitors to achieve the desired particle shape.

Other types of reactors which show varying types of flow to introduce agaseous reactant into a slurry are exemplified by U.S. Pat. Nos.2,000,953; 2,704,206; 3,045,984; 3,417,967; 3,856,270; 4,313,680; and4,514,095. All of the foregoing reactors use complex mechanisms toprovide a motion or direction change to a slurry moving through thereactor to enhance gas-liquid contact.

There is a need to provide for both improved processes for gas liquidcontacting and improved apparatus that can be fabricated easily andeconomically to carry out such processes.

SUMMARY OF THE INVENTION

The present invention pertains to a method and apparatus for improvingcontact between a liquid and a gas, either or both of which may be areactant. The process of the present invention involves causing theliquid to move in a serpentine path through a reactor so that theserpentine path causes the liquid to move both laterally and verticallyas the liquid proceeds from one station, section, stage, zone, orchamber to another in a novel reactor. As the liquid moves in theserpentine path gas is introduced below the surface of the moving liquidat, at least one, but preferably many locations in each zone. Thereactor according to the present invention is designed to effectmovement of the liquid in the horizontal and vertical serpentine motion(tortuous flow) through discrete chambers in the reactor. Gas can beintroduced into the liquid in any one or all of the chambers.

The number of chambers in a reactor can be constructed in a single lineor in banks of rows arrayed side-by-side and reactors can be gangedtogether in various lateral, or nested arrays in order to achieve therequired gas liquid contact. In point of fact the chambers can bearranged in any configuration to accommodate the constraints of aparticular plant layout, as long as the flow path is as describedbetween the chambers. Thus a reactor according to the invention can haveany number chambers arranged in any number of rows inside a givenreactor. The reactor can be multiple reactors or modules connected inseries to achieve an overall reactor of any required length that definesa continuous flow path.

Therefore, in one aspect the present invention is a continuousgas-liquid contact reactor comprising in combination; an elongatedhousing having the general shape of a four sided polygon, the housingadapted to contain a bath of liquid, a plurality of individual chambersdisposed within the tank, the chambers arranged to permit the liquid toflow sequentially from a first chamber to a last chamber, means tointroduce the liquid into the first chamber and withdraw liquid from thelast chamber, means in the housing to direct the liquid from a point ofentry in each chamber, being one of at a top corner or a diagonallyopposed bottom corner, in a general direction to point of entry into thesucceeding chamber which is diametrically opposed to the point of endingfrom the previous chamber, and means to introduce a gas, optionally areactant gas, into one or more of the chambers below the level of liquidflowing through the chamber. When the gas is not a reactant gas, theliquid is typically composed of merge streams of reactants. In anotheraspect the present invention relates to a method for enhancing contactbetween a liquid and a gas, e.g. a reactant gas, comprising the stepsof; moving the liquid along a confined path from a point of entry to apoint of exit in a generally elongated vessel, the liquid caused to movein a generally serpentine path through a plurality of stages or chambersin the vessel, the serpentine path being defined as causing the liquidto move laterally and alternately from top to bottom or from the bottomto the top in each of the chambers, and introducing the gas into theliquid in at least one of the chambers through which the liquid ismoving.

The present invention includes a further optional method step ofrecycling gas, such as the unreacted collected reactant gas back to theliquid or some other part of an overall process. For example in themanufacture of precipitated calcium carbonate, carbon dioxide escapingfrom the bath, where it reacts with the calcium hydroxide in the water,could be collected and recycled to the compressor, blower, or fan usedto introduce fresh carbon dioxide into the process.

In still another aspect the present invention is a precipitated calciumcarbonate having any of the known crystalline structures, for example, acalcitic or aragonitic crystalline structure or mixtures of bothcalcitic and aragonitic precipitated calcium carbonate, made by reactinga liquid containing calcium hydroxide and water with a reactant gascontaining carbon dioxide produced by; moving the liquid along aconfined path from a point of entry to a point of exit in a generallyelongated vessel; the liquid caused to move in a generally serpentinepath through a plurality of stages or chambers in the vessel, theserpentine path being defined as causing the liquid to move laterallyand vertically in each of the chambers, and, introducing the reactantgas into the liquid in at least one of the chambers through which theliquid is moving.

The present invention also pertains to a method of producing aprecipitated calcium carbonate with a controlled crystalline structureby contacting a liquid containing calcium hydroxide and water with areactant gas containing carbon dioxide comprising the steps of; movingthe liquid along a confined path from a point of entry to a point ofexit in a generally elongated vessel, the liquid caused to move in agenerally serpentine path through a plurality of stages or chambers inthe vessel the serpentine path being defined as causing the liquid tomove laterally and vertically in each of the chambers, and introducingthe reactant gas into the liquid in at least one of the chambers throughwhich the liquid is moving.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1, is a schematic isometric representation of liquid flow through aportion of a reactor according to the present invention.

FIG. 2, is a schematic isometric drawing of a reactor according to thepresent invention illustrating one arrangement for the various chambersor sections of the reactors tank.

FIG. 3, is a schematic front elevation of the apparatus of FIG. 2according to the present invention.

FIG. 4, is a top plan view of the apparatus of FIG. 3 with the cover andexhaust system, and fluid recirculation system not shown.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the basic structure of the reactor 10 is agenerally elongated tank 12 having ends 14, 16 and sides 18 and 20.While the reactor tank 12 can have the configuration of any four sidedsolid polygon having a generally square or rectangular longitudinalcross-sectional shape is preferred. The tank 12 is provided with aninlet 31 for fluid, represented by arrow 29, and exit conduit 32 forwithdrawal of the treated product, represented by arrow 33. Tank 12includes a plurality of internal baffles 22, 24, 26, 28 and 30 spacedthroughout length of the tank 10 to divide the tank into six chambers(modules, sections, stages, compartment, zone, etc.) of approximatelyequal size. The spacing of the baffles 22, 24, 26, 28 and 30 can berandom so that the chambers are of varying size or spaced equally tocreate chambers of equal size. A reactor according to the presentinvention can contain any number of chambers either arrangedlongitudinally or in side-by-side rows, the number of chambers in a rowor bank determined by the process to be carried out in the reactor. Thevarious figures of the drawing show different numbers of chambersarranged in side-by-side rows for purposes of illustration andexplaining the invention. The total number of chambers in any reactorcan vary from two to a number defined as N, the total number, as statedabove determined by the process for which the reactor is to be used.Baffles 22, 24, 26, 28 and 30 have passages 35, 34, 36, 38 and 40respectively which are placed at either an upper portion of the baffleas shown by passage (port or aperture) 35 (baffle 22 ) or the oppositebottom corner of the succeeding baffle- such as shown with passage 34(baffle 24 ). In the balance of this specification, the invention may bedescribed in terms of the use of a reactant gas, although it is to beunderstood that the description applies equally well to merged reactantstream and a non-reactant gas unless the description context limitsotherwise.

In the schematic of FIG. 1 fluid, represented by arrow 29, introducedthrough inlet conduit 31 is conducted to the bottom of the reactor 12and begins a path from a first chamber, compartment or zone of thereactor 12 to the next in series from the front wall 14 to the back wall16 as shown by arrows 40, 42, 44, 46, 48, 50, 52, 54, 56, 58 60, and 62respectively. It is within the scope of the invention to have the fluidentry at any location between the top and bottom of the reactor. Asshown by the arrows the fluid generally moves from the bottom of onechamber to the top of that chamber and out the passage down through thenext chamber and exits the bottom of the succeeding chamber thusdefining a serpentine path with the serpentine moving both verticallyand horizontally as the fluid flows through the reactor 12 as shown inFIG. 1. This might also be called tortuous flow of the fluid through thereactor from the inlet 31 to the outlet 32.

FIG. 2 shows a reactor 70, which has a generally longitudinalcross-sectional rectangular shape with a front wall 72, a back wall 74,a side wall 76 and an opposite side wall 78. The reactor 70 alsoincludes a longitudinal baffle 80 which extends unbroken from the frontwall 72 to the back wall 74 of the reactor. Longitudinal baffle 80includes a cross flow passage 82 the purpose of which will behereinafter explained.

Reactor 70 also includes a series of transverse vertical baffles 84, 86,88, 90, 92, 94, 96, 98, 100, 102, 104, 106, 108, 110 so that with thefront wall 72 and the back wall 74, reactor 70 is divided into 16separate compartments. An inlet conduit 112 communicates with thechamber defined by baffle 84, longitudinal baffle 80, wall 72 and wall76. An exit conduit 114 communicates with the chamber defined by baffle110 wall 78 longitudinal baffle 80 and wall 72. The internal baffles 84,86, 88, 90, 92, 94, 96, 98, 100, 102, 104, 106, 108 and 110 are allfitted with alternating passages such as shown by the dotted lines sothat the fluid flow from the first chamber having the inlet conduit 112to the last chamber having the outlet conduit 114 is in a flow patternsuch as shown in FIG. 1. Thus a reactor can be made of any length or canreturn on itself as shown in FIG. 2 to enable a user to make a reactorof shorter overall length using forward and reverse flow paths thuspermitting installation of a reactor in a confined space. A reactor suchas shown in FIG. 2 can be connected to another reactor that is of adifferent configuration, e.g. a different number of chambers, or isidentical to reactor 70 so that the outlet 114 is connected to the inletof the second reactor (not shown).

A reactor according to the invention can have, as stated above, anynumber of chambers of varying dimensions arranged in a single row or anynumber of side-by-side rows in a unit or module. A reactor can byachieved using a single unit or module or a number of units or modulesconnected in series. If there are no space constraints the reactor canbe constructed with all chambers in a single row thus defining a modulewhich is also the reactor. However, if there are space limitations wherethe reactor is to be installed, the reactor can be fabricated in moduleswhich are then connected in series to define a continuous flow paththrough the reactor. In this case each module can have a given number ofchambers in a row with the rows arranged in a side-by-sideconfiguration. The modules can be arranged in horizontal, vertical, ormixed horizontal and vertical arrays as long as the flow path througheach module and through the reactor is as taught herein.

Referring to FIG. 3 and FIG. 4, the reactor 70 is shown with two rows(FIG. 4), each row having nine zones or chambers defining a reactor with18 zones in a 30 bank. The reactor 70 is fitted with a removable cover118 and with a header pipe 120 from which depend or project individualreacting gas introduction pipes 122, 124, 126, 128, 130, 132, 134, 136and 138 in one side or line of one bank and a complimentary set ofdepending or projecting gas introduction pipes (123, 125, 127, 129, 131,133, 135, 137, and 139) in the other side or line of the bank is shownby the dots in FIG. 4.

The conduit 120 is used to introduce a reactant gas represented by arrow140 into the liquid. Since the dependent supply pipes, e.g. 122 extendbelow the level of the liquid which is indicated at 142 the gas isintroduced into the liquid in an individual chamber. One or all of thegas introduction pipes may be used depending upon the nature of thereaction desired and the material being contacted. As shown in FIG. 4, asingle inlet pipe can be manifolded to individual supply pipes 144 and146 to introduce the reactant gas into the liquid. Reactant gaspermeating (escaping) from the liquid 143 can be collected using acollection conduit 148 which in turn is connected to a pump or otherevacuation device 150 which produces an effluent 152 which can befurther processed to reclaim a reactant gas or can be directed for useas reactant gas in some other part of the process or for temperature,pressure, or composition control in some other part of a larger overallprocess scheme, the use depending upon the gas liquid contact reactionbeing effected. It is also possible to collect and recycle the gas to bemixed with fresh reactant gas introduced into the process. For example abranch conduit (not shown) could be fitted into conduit 148 and anauxiliary fan could be used to withdraw and recycle the exhaust gas. Itwould also be possible to use the exhaust device 150 for recycling theexhaust gas.

Fluid inlet shown in FIG. 3 is represented by arrow 154 and fluid exitby arrow 156 (FIG. 4). Optionally a recycle loop 158 comprising awithdrawal conduit 160, recirculating pump 162 and delivery conduit 164can be included in the system and can be placed in any of the chambersto withdraw liquid and recycle it to any other chamber i.e. from themiddle of the reactor or gas liquid contactor to the entry or firstmodule or chamber. It is also possible to have multiple recirculationlines or conduits between chambers in order to effect the overallprocess and final product quality. The depth of liquid in each chamber,while shown as uniform for the purpose of illustration, is notnecessarily the same. Depending upon the design, (e.g. shape,dimensions, spacing from the bottom of the reactor), of the notches orpassages in each baffle the level of the liquid can vary between eachchamber and be greater in the first zone or chamber than in the lastzone or chamber.

Thus, the process of the present invention utilizes the reactor shown topump a liquid reagent into the feed or first end of the vessel (e.g. 70)with a reactant gas introduced through the various lances 122, et. seq.The liquid flows through the various chambers (zones) in a sequentialflow pattern (left to right in the drawing), diagonally across eachchamber in a serpentine (alternating over/under) pattern or flow whichmaximizes gas-liquid contact and aides in the mixing and transport ofthe liquid and solid particles contained in the liquid to be reactedwith the reactant gas introduced into the liquid.

Gas permeating from the liquid 143 is captured in the top of the vesselor reactor 70 because of the cover or top 118. The combination of anevacuating (ventilating) pump 150 and the top 118 creates a dynamic sealand prevents gas infiltration from the reactor into the surroundingatmosphere.

On initial startup the reactor 70 is filled with a liquid reactant, e.g.water, so that it overflows the outlet conduit 156. At this point, gasflow is initiated and the reactant material is introduced into the inletof the reactor as shown by arrow 154. Gas is delivered by a smallcompressor, blower or fan through the piping as shown, from a source(not shown) which may be an on-site waste stream or the like. However,it is also within the scope of the present invention to provide a directsource of reactant gas from high pressure storage devices such ascylinders, tubes or direct vaporization of gas stored as a liquid.

A reactor according to the present invention can be constructed so thatthe average depth of liquid in the reactor ranges from about 1 inch toabout 360 inches.

It is also possible to take the gas coming out of the reactor viaconduit 152 and use it in a downstream process to correct or controlprocess conditions such as temperature, pressure and/or pH or to recoverheat from the exhaust in gas for reuse in the process, or to recycle theexhaust gas back to the process to obtain maximum utilization of theprocess.

A reactor according to the present invention was used to produceprecipitated calcium carbonate for use as a paper brightening agent. Asis well known in the trade precipitated calcium carbonate can beproduced in various particle shapes (morphologies) depending upon thepaper to which it is applied and the requirements of the paper mill.

The reactor of the invention can also be used to produce fillers forpaper making and liner-board manufacturing, as well as non-paperapplications such as plastics, sealants, and other users of precipitatedcalcium carbonate.

A reactor according to the present invention was constructed and tested.The reactor had overall inside dimensions of, 7.3 ft (length) by 9.25inches wide with fourteen chambers or zones. The chambers wereconstructed with passages between each chamber as shown in the drawingso that a nominal depth of three feet of liquid was maintained in thereactor. The reactor was arranged so that zones 1 through 4 were 2.625inches long, zone 5-13 where 7.25 inches long and zone 14 was 11.625inches long. The reactor contained a single line or row of chambers,however as explained above and shown by the test results below, variousconfigurations of the chambers or chamber modules may be used.

Table 1 sets forth a comparison of target conditions and an actual runfor the reactor described above.

TABLE 1 Actual Pilot Conditions Run Number — 4 Gas Temperature ° F.68.00 Gas Pressure Basis psig 0.00 Total Gas Flowrate cfm 60.00 Gas CO₂Concentration vol % 15.00 CO₂ Flowrate cfm 9.02 CO₂ Flowrate lb/min 1.03Gas Efficiency % 51.58 PCC Rate lb/min 1.21 PCC Rate lb/h 72.45 AverageSlake MO ml-1N- 7.90 HCl Slake Feedrate gpm 1.83 SSA m²/g 4.40 ProductMorphology — Aragonite Reactor Total Volume gal 122.55 Number of Zones —14.00 Zone volume, Numbers 1-4 gal/zone 3.68 Zone volume, Numbers 5-13gal/zone 10.17 Zone volume, Number 14 gal/zone 16.31

The data set forth in Table 1 show that a reactor according to thepresent invention can be used to produce a precipitated calciumcarbonate (PCC) with a defined crystalline structure. The actual reactorconditions were close to those or exceeded those that were targeted.Under actual test conditions the reactor according to the inventionshowed improve productivity over that which was targeted. A continuousreactor provides higher availability and can be smaller than a batchreactor, thus reducing capital costs to the user.

The present invention has been described in relation to the manufactureof precipitated calcium carbonate. However the method and apparatus ofthe invention can be used in other applications where a gas isintroduced into a liquid for reaction with the liquid or components inliquid.

For example the present invention would be applicable to treatment ofsewage by moving liquid sewage through the reactor and introducing anoxidant, e.g. air, oxygen or both, through the gas induction pipes.

Iron particles in a solution could be oxidized to various iron oxidecompounds using the method and apparatus of the invention.

In another application liquids could be treated with a reactant such ashydrogen chloride where air is introduced into the gas introductionpipes to aid in suspension and transport through the reactor, forexample in the following reactions:C₅H₁₁OH+HCl=C₅H₁₁Cl+H₂OHCl+NaOH=NaCl+H₂OFe+2HCl=FeCl₂+H₂CaCO₃+H₂SO₄=CaSO₄+H₂O+CO₂

The method and apparatus of the invention can be used to effectgas/liquid reactions where a mixture of a reactant gas (e.g. CO₂) andair are used for suspension and transport through the reactor. Examplesof such reactors are:NaOH+CO₂=NaHCO₃2NaOH=CO₂=Na₂CO₃Ca(OH)₂+CO₂=MgCO₃+H₂OMg(OH)₂+CO₂=MgCO₃+H₂O

Thus a reactor according to the present invention which may bedesignated a horizontal, open channel, plug flow reactor can be used tomatch or exceed throughput of a batch gas liquid reaction. The reactoraccording to the present invention does not require a pressure vesseland does not require mechanical agitation thus eliminating the need forexpensive motors. Motors can increase capital, maintenance, andoperating costs for a conventional continuously stirred tank reactor ora batch reactor system.

A reactor according to the invention described herein can provide a costeffective way to produce products such as precipitated calcium carbonatewith high solids concentration.

It is also within the scope of the herein described invention to use thereactor to produce other products where a gas-liquid reactor isrequired.

1. A method for precipitating calcium carbonate from a liquid containingcalcium hydroxide and water comprising the steps of: providing acontinuous flow of the liquid along a serpentine path sequentiallythrough a plurality of chambers in a vessel having an inlet and anoutlet, each of the plurality of chambers having a point of entry and apoint of exit positioned diagonally opposing one another defining theserpentine path, and the plurality of chambers being arranged to permitthe liquid to flow sequentially from the first of the chambers to thelast of the chambers, said arrangement being defined by the point ofexit of one chamber being connected to the point of entry of thesubsequent chamber, wherein the point of entry of the first chamber isthe inlet of the vessel and the point of exit of the last chamber is theoutlet of the vessel; and introducing a reactant gas into the liquid inthe plurality of chambers, the reactant gas being introduced via asingle supply pipe which is manifolded to individual supply pipes thatextend into respective ones of the plurality of chambers, the reactantgas being introduced into the liquid in the plurality of chambers viathe individual supply pipes extending below the level of the liquid,wherein the liquid and the reactant gas react to form precipitatedcalcium carbonate without any mechanical agitation.
 2. The methodaccording to claim 1, wherein the reactant gas contains carbon dioxide.3. The method according to claim 1, further comprising the step ofrecovering reactant gas escaping from the liquid.
 4. The methodaccording to claim 3, further comprising the step of recycling therecovered reactant gas.
 5. The method according to claim 1, wherein theliquid is maintained at an avenge depth of at least about 1 inch ofliquid in the vessel.
 6. The method according to claim 1, wherein theliquid is maintained at an average depth of from about 36 inches toabout 360 inches of liquid in the vessel.